In a pipe line in the conventional semiconductor manufacturing process and so forth, a sub-line 32 is installed as necessary and a valve 33 is provided for the adjusting or opening and closing control of the fluid being supplied to said sub-line 32, as shown in FIG. 6. If the valve 33 is closed, the fluid in the line 35 from a branch point 34 to the valve 33 remains stagnant as a necessary consequence. If this stagnant state is continued for a long period of time, microorganisms and the like develop in the fluid and the purity of the water deteriorates. As one of the means for solving these problems, a method that includes minimizing the volume of line 35, which is the stagnant portion, for instance, using a branch valve as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-151676 in the branch point 34 can be considered.
However, because the valve 33 must be positioned close to a main line 31 according to said conventional method, its mount position is restricted, thereby creating serious problems in such a complex piping system as that inside the unit. Further, when using said branch valve as the valve 33, the valve is not only subjected to the aforesaid restriction but also if the caliber difference between the main line 31 and the sub-line 32 is substantial, the fabrication of the branch valve itself may be difficult and it is impossible to completely eliminate this stagnant portion, in view of its structure, because, if a ball valve is used as a branch valve, the fluid sealed between the valve chest space sealed by two seat rings inside the ball valve remains stagnant when the ball is in a closed position. What in more important is that the entire line 37 from said valve 33 to a use-point 36 becomes a stagnant portion if the valve 33 is closed.